The electric dust precipitator, in principle, comprises a pair of opposite dust-collecting electrodes and a discharging wire disposed intermediately therebetween, which are positive and negative electrodes, respectively. When a high direct current voltage is applied between the electrodes and the wire, the electric field generated near the discharging wire is distorted so that a negative corona discharge may take place.
When a gas to be treated, for example, a waste gas containing smoke dust, is fed through the space between the dust-collecting electrodes and the discharging wire, i.e., through the discharging region, dust particles in the gas are negatively charged and then attracted to the dust-collecting electrodes.
The method for removing fine particles in the gas in accordance with the above-described principle is known to be highly effective for dust precipitation.
All the prior electric dust precipitators depend on the above principle, though modifications and variations are, of course, made in practice. One of the improvements is made in accordance with the removal of the dust precipitated on dust-collecting electrodes. Usually, the collected dust is removed in a mechanical manner. One method uses a hammer which shocks and vibrates the dust-collecting electrodes to knock off the collected dust. An alternate method uses a slider or scraper which moves along the surface of the dust-collecting electrodes to scrape off the dust.
The former method, however, has serious drawbacks. First of all, a shock wave caused by the hammer is so violent that beams supporting the electrodes tend to fail. Particularly, if the dust-collecting electrode is of a tubular type through which cooling water is passed, failure or cracks in the electrodes or joints between the electrodes and the beams (or headers) may result in leakage of water. Besides, the shock given by the hammer is partly absorbed by the cooling water within the tubular electrodes, resulting in insufficient vibration of the electrodes. Further, it is difficult to effectively knock off highly adhesive dust even at more accelerated vibrations of the hammer. Such insufficient removal of dust necessitates periodic suspension of operation to clear the electrodes.
In addition, the dust which has adhered to the electrodes instantaneously drops as soon as the hammer makes impact with the electrodes. Upon settling to the bottom, the dust disperses again, and as a result, dust in the discharging space between the dust-collecting electrodes instantaneously becomes a concentration several tens to several hundreds times higher than usual. The corona discharge in the relevant space is suppressed by such higher concentrations of dust that the dust-collecting capacity is reduced to a large extent, and the concentration of dust at the exit of the precipitator is temporarily increased.
The latter method which uses a sliding scraper can overcome the above-described drawbacks inherent in the hammering method, but it has its own disadvantage. The prior scraper is moved up and down in a sliding manner against the surface of the dust-collecting electrodes. The dust is not completely scraped off, however. Particularly when the sliding scraper is to be moved during operation, dust also precipitates onto the back of the scraper. Consequently, the operation must be suspended to remove the dust precipitated onto the back.